[pve-devel] Fwd: Re: recommanded cache setting for rbd image

[Dietmar Maurer]

> >>block: default cache mode is now writeback
> >>
> >>so it is already 'writeback'?
> I think we need to read the qemu code. I don't know if writeback is writeback
> mode in guest or writeback in host.
> cache=none : writeback is guest
> cache=writeback : writeback is guest,writeback in host

So far, we only talk about cache mode in the host. I do not care what the guest it doing ;-)

> One another interesting thing in qemu 1.2 changelog:
> 
> "Emulated IDE and SCSI as well as virtio-blk devices can now switch the cache
> mode between writethrough and writeback. virtio-blk automatically switches
> to writethrough if the guest driver doesn't support flushes. "
> 
> So maybe if we set cache=none and that guest doesn't support flushes
> (drivers,kernel,?), it become writethrough in guest, so it's safe ?

No, default cache is 'writeback'. But this is unsafe when the guest doesn't support flushes.
That is why it is changed to writethrough to be safe.

But why does a guest do not support flushes? What kind of guest is that?